# A level Trigonometry Help

Watch
Announcements
#1
Express 4 sin x + 3 cos x in the form r sin (x + α). Hence find all the values of x in the range 0 ≤ x ≤ 360° for which cos 3x = cos 2x.

I‘ve done the first part, which I got 5 sin (x + 36.9°), but I have no idea how to solve the second part.

0
4 weeks ago
#2
Can you upload a picture of the question?
1
#3
0
4 weeks ago
#4
I think they want you to.expand the multiple angles in terms of cos(x) and sin(x).
However, id just take acos directly and forget about the first part.
Last edited by mqb2766; 4 weeks ago
0
#5
(Original post by mqb2766)
I think they want you to.expand the multiple angles in terms of cos(x) and sin(x).
However, id just take acos directly and forget about the first part.
I‘ve tried to expand it in terms of cosx and sinx but I can’t link back to the first equation.
0
4 weeks ago
#6
Cos(A+_B) = CosAcosB + SinASinB
0
4 weeks ago
#7
(Original post by charlenecsn)
I‘ve tried to expand it in terms of cosx and sinx but I can’t link back to the first equation.
0
#8
(Original post by CaptainDuckie)
Cos(A+_B) = CosAcosB + SinASinB
I got 4cos^3(x)-3cos(x)=2cos^2x-1, but I can't relate this to the first equation.
0
4 weeks ago
#9
0
#10
(Original post by mqb2766)
0
4 weeks ago
#11
Cannot see that at all😂😂
0
#12
(Original post by CaptainDuckie)
Cannot see that at all😂😂
you mean the pic cant be viewed?😂
0
4 weeks ago
#13
(Original post by charlenecsn)
you mean the pic cant be viewed?😂
0
#14
(Original post by CaptainDuckie)
0
4 weeks ago
#15
(Original post by charlenecsn)
Will have a look in the morning, but the simple thing to do is take acos
to give
3x = +/-2x + 2*pi*n
0
#16
(Original post by mqb2766)
Will have a look in the morning, but the simple thing to do is take acos
to give
3x = +/-2x + 2*pi*n
Alright, thank you so much!!!
1
4 weeks ago
#17
As a side note, another method is to use the trigonometric sum-product results, specifically:

.

For this case: .
0
#18
(Original post by simon0)
As a side note, another method is to use the trigonometric sum-product results, specifically:

.

For this case: .
Thanks! ))but that formula isn't in the syllabus
0
4 weeks ago
#19
(Original post by CaptainDuckie)
Cos(A+_B) = CosAcosB + SinASinB
Close but not quite.
0
4 weeks ago
#20
(Original post by simon0)
Close but not quite.
It’s the Sin(A _B) = SinASinB - CosAcosB

( I realised )

But forgot to edit
Last edited by CaptainDuckie; 4 weeks ago
0
X

new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### Poll

Join the discussion

#### Are you travelling in the Uni student travel window (3-9 Dec) to go home for Christmas?

Yes (99)
28.37%
No - I have already returned home (45)
12.89%
No - I plan on travelling outside these dates (67)
19.2%
No - I'm staying at my term time address over Christmas (37)
10.6%
No - I live at home during term anyway (101)
28.94%